The combustion of fossil fuels in vehicles for the reason of transportation is the single largest source of human-made greenhouse gases. Scientific evidence strongly supports the hypothesis that the emission and accumulation of these greenhouse gases in the atmosphere is raising the earths average temperature and impacting the overall climate.
To assess transportations role in global warming and air pollution, emissions of greenhouse gases for a particular fuel source should be measured using the full fuel-cycle estimates. These estimates measure the total emissions of CO2-equivalent greenhouse gases primarily carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from wells to wheels and the vehicle cycle through material recovery and vehicle disposal needs. This allows for a more accurate comparison between fuel sources in estimating the over pollution impact on the environment.
These CO2-equivalent greenhouse gases are described as follows:
- Carbon dioxide released during the combustion of solid waste, wood, and fossil fuels. Twenty-percent of the total CO2 emissions in the United States come from cars and light trucks.
- Methane emitted during the production and transport of coal, natural gas, and oil. Other sources may include the decomposition of organic wastes in landfills and the raising of livestock.
- Nitrous oxide emitted during agricultural and industrial activities, as well as during the combustion of fossil fuels. Nitrous oxide plays a key role in the disruption of the ozone cycle, producing elevated level of ozone in the troposphere.
The full fuel-cycle estimates also consider the emissions of five criteria pollutants: volatile organic compounds (VOCs), carbon monoxide (CO), nitrogen oxide (NOx), particulate matter size smaller than 10 microns (PM10), and sulfar oxides (SOx). These criteria pollutants are described as follows:
- Volatile organic compounds Organic compounds that evaporate readily into the atmosphere at normal temperatures. VOCs contribute significantly to photochemical smog production and certain health problems.
- Carbon monoxide forms when carbon in fuel is not completely combusted. The EPA estimates that in major cities as much as 95 percent of carbon monoxide is released from mobile sources. Carbon monoxide exposure can reduce the amount oxygen delivered to organs and tissues, and can be harmful to those suffering from heart and respiratory disease.
- Nitrogen Oxide are formed when fuels burn at high temperatures, such as in motor vehicles. Mobile sources produce more than 50 percent of all nitro oxide emissions in the United States, one of the largest contributors to smog formation in large cities.
- Particulate Matter consist of minute particulate matter or liquid droplets in the air that contains a variety of chemical components. Larger particulate matter may settle out more rapidly than small matter. Diesel-powered vehicles and engines contribute more than 50% of the particulate matter emitted in the United States. Particles less than 10 microns in diameter pose a significant health risk and have been linked with illnesses and deaths from heart and lung disease.
- Sulfur Oxides formed when fuel containing sulfur is burned during metal smelting and other industrial processes. Transportation sources contribute less than 10 percent of sulfur oxides emitted into the atmosphere. Exposure to sulfur oxides may cause wheezing, chest tightness, or shortness of breath. Long-term exposures may result in aggravation of existing cardiovascular disease or respiratory illnesses.
The Bush Administration has rejected participation in the Kyoto protocol a protocol, if signed, that would reduce greenhouse gas emissions by at least 5 percent below 1990 levels in the commitment period 2008 to 2012. This reluctance to participate in the Kyoto protocol has drawn sharp criticism from ranking members of Congress.